Enhanced Data rates for GSM Evolution (EDGE) is an enhancement of the General Packet Radio Service (GPRS) radio interface in GSM. Nine Modulation and Coding Schemes (MCSs) are defined in EDGE. The lowest MCSs have lower bitrates but are more robust while usage of higher MCSs enables higher bitrates and increased capacity. EDGE also supports Incremental Redundancy (IR) which is a technique for combining transmissions of the same data to increase the decoding success rate.
EDGE Evolution is currently being standardized in the Third Generation Partnership Project (3GPP) Rel-7. The work items include, amongst other things, higher order modulation, dual carrier transmission downlink, reduced latency and dual-antenna terminals. The reduced latency work item includes reduced Time Transmission Interval (TTI), fast Acknowledgement/Negative Acknowledgement (Ack/Nack) reporting and Radio Link Control (RLC) non-persistent mode. Reduced latency is vital for delay sensitive applications like Voice over Internet Protocol (VoIP), which is a system enabling voice data to be delivered using the IP.
Usually a conversational service, such as VoIP, will have a mouth-to-ear delay requirement in order to assert an acceptable conversation quality. In a cellular system the mouth-to-ear delay basically includes one delay in the transmitting link, from the user equipment (UE) to the Base Station Controller (BSC) uplink (UL), and one delay on the receiving link, from BSC to UE downlink (DL). VoIP usually has a pre-defined delay budget e.g. 300 ms mouth-to-ear. All packets then need to be transferred within this delay budget.
With reduced latency the VoIP delay budget may include and be limited to one RLC retransmission on both uplink and downlink directions, called RLC non-persistent mode. This increases the coverage of the VoIP application since IR increases the decoding success rate for a retransmission. The capacity is also increased since modulation and coding schemes, MCS, carrying larger payload may be used.
The enabling of one RLC retransmission on both DL and UL increases the coverage and capacity of the VoIP service. However, if two retransmissions could be utilized in either link then the gain would be even higher. If information about the age of each Protocol Data Unit (PDU) conveying speech samples can feasibly be provided to the entity controlling the radio link at the receiving end, it could be taken advantage of as outlined above.
However, GSM/EDGE Radio Access Network (GERAN), where the BSC/Packet Control Unit (PCU) controlling the radio link entity at the receiving end does not know the status of the VoIP data that arrives, another approach to estimate the state/age of the VoIP data is required.
Thus, a generic solution for communication systems where the explicit information about the age of the PDUs conveying the VoIP data is not available to the radio link entities at the receiving ends that are utilizing a limited number of retransmissions over the air interface is needed.